Wow that is some fuel you are using. I have an econoline that would give you a run for the money.
I would guess with 14 mpg (US) it might be an accumulation of several things. Some things to check (that I don't think have been mentioned) are a fuel leak, tire pressures and how you drive it?
Replace your cap and rotor, you will thank me
had about 14mpg in my manual golf. It ran OK with the screwed up cap and rotor but boy that idle noise quieted down . Sounded less like a nice chug and more like a vw should